Melted Rocks (Clinkers and Paralavas) from the Khamaryn–Khural–Khiid Combustion Metamorphic Complex in Eastern Mongolia: Mineralogy, Geochemistry and Genesis

The mineralogical and geochemical features of melted rocks of the Khamaryn–Khural–Khiid combustion metamorphic complex in Eastern Mongolia have been characterized for the first time and conditions of their formation have been estimated. Clinkers and paralavas were formed through partial melting of Early Cretaceous sedimentary sequence under the effect of wild coal fires, which have happened since at least 19th century. The studied area of the complex is dominated by pyrogenically modified pelitic rocks, while their melted varieties represented by clinkers of dacitic and rhyolitic compositions are restricted to the coal fire epicenters. Much less common are Ca-rich and Na-bearing silica-undersaturated mafic paralavas formed through melting of silicate minerals (newly formed and relict) in limestone. Pyrogenically modified mudstones and clinkers contain felsic silicate glass with melted grains of quartz, feldspar, monazite, and other minerals, as well as newly formed cordietite–sekaninaite, hercynite, magnetite, hematite, goethite, ferrosilite, cristobalite, barite-celestine, and accessory rutile, ilmenite, fluorapatite, and fayalite. The paralavas are fully crystallized rocks consisting of phenocryst assemblage (plagioclase, Al-clinopyroxene, melilite) and interstitial pleonast, K–Ba feldspar (celsian, hyalophane), minerals of the rhonite-kuratite series, Ca–Fe olivines (Ca-fayalite, kirschsteinite), pyrrhotite, barite, and fluorapatite. There are also barium Fe–Cu sulfides of the djerfisherite (zoharite, owensite) group and Fe phosphides (schreibersite, barringerite, and α-Fe + Fe3P eutectic). At the final stage, Ca-bearing nepheline-group mineral, supposedly, davidsmithite, crystallized in paralava matrix. The paralavas crystallized under high-temperature conditions (1365–945°C for Fe phosphides, near 1100°C for minerals of the rhonite-kuratite series, and 950–850°C for Fe–Ca olivines) at oxygen fugacity from IW to QFM buffers. Locally developed reducing conditions provided the formation of small drops of Fe phosphides and native iron. Some paralavas contain Fe3+-pleonast and rhonite-subgroup minerals with high dorrite end-member fraction, which were formed in oxidizing conditions likely corresponding to the HM buffer. Melilite–nepheline paralavas of combustion metamorphic complexes of Mongolia (Khamaryn–Khural–Khiid and Nyalga ones) have close mineralogical and geochemical features. Similar paralavas have not been described anywhere else in the world. The differences in the mineral–phase assemblages and geochemical composition of the clinkers and paralavas from these complexes can be related with composition variations of sedimentary protolith (pelitic and carbonate rocks) involved in the partial melting, as well as with local variations in the physicochemical conditions of their formation (temperature, fluid regime, oxygen fugacity, cooling–quenching rates of the pyrogenic melts, and other factors) during wild coal fires.